Box's alpha parameter

[PierreGtch]

Hi, thanks for this great library!

Right now, the transparency parameter alpha is not automatically passed to the box plot and we can not pass it via box_alpha because it has to be done via the boxprop parameter (see https://github.com/mwaskom/seaborn/issues/979#issuecomment-235082271). I think it would be nice to also set the boxes transparency via alpha.